In the manufacture of paper and board products, a highly aqueous stock consisting of about 98-99.8% water and from 0.2-2% papermaking fibers and other solids is ejected at high speed from a headbox slice onto a moving forming fabric. Adjacent the head box slice, the forming fabric passes in sliding contact over a plurality of static fabric support elements which serve to support the forming fabric, and to define a reference surface over which the forming fabric moves. Depending on the surface profile chosen for the fabric support elements, they may also assist in draining water from and generating turbulence in the stock on the forming fabric. The fabric support surfaces usually include a lead blade located more or less underneath the point at which the stock jet impinges the forming fabric, followed downstream by at least two additional surfaces, each of which may be flat, or profiled to act as foils (e.g. as disclosed by Wrist in U.S. Pat. No. 2,928,465) or as agitators (e.g. as disclosed by Johnson, in U.S. Pat. No. 3,874,998), although stock agitation is not typically initiated at this very early point in a forming section. In current practice, the individual fabric contacting elements adjacent the head box slice are not vertically adjustable after installation (see for example Rulis et al., U.S. Pat. No. 6,274,002). These elements are normally mounted onto the supporting structure using either a dovetail, or a T-bar, and in some cases are solidly mounted. The mountings are also arranged on the supporting structure so that all of the elements are in permanent non-adjustable contact with the forming fabric. Stock activity can only be adjusted by changing, or physically removing, the fabric contact elements by sliding them out of their dovetail or T-bar mount. It is both very difficult and time consuming to carry out such a change in the configuration of the forming section fabric supporting elements while the paper making machine is running, bearing in mind that the forming fabric can be moving at up to 100 kph.
Good sheet properties result from the injection of kinetic energy into the stock, which causes the papermaking fibers to become agitated and thus relatively more uniformly dispersed. This is particularly important in the early part of the forming section adjacent the head box slice, where the paper making fibers are still relatively mobile.
Paper makers are currently seeking means to accommodate changes in the basis weight of the paper product being manufactured by the same machine. For example, a papermaking machine producing liner or board grades at a relatively low basis weight may have to shift to a far higher basis weight product which requires a much slower machine speed. This change in speed reduces the beneficial stock turbulence and adversely affects product quality. It is thus apparent that a given papermaking machine, without some means of adjustment, is best at making one grade of paper product and cannot readily be altered to make a significantly different one. It is not uncommon today for a papermaking machine to have to accommodate a doubling or tripling of the basis weight of the product being manufactured. Such changes are frequently difficult to manage without compromising sheet quality, and therefore alternative means of achieving a required level of activity in the stock are necessary to accommodate changes in paper grade with minimal disruption to production.
This invention seeks to provide a method and an apparatus by means of which the paper maker can alter the number of fabric support elements in contact with the forming fabric in the area immediately after the stock impinges onto the forming fabric adjacent to the head box slice so as either to enhance, to maintain or to diminish stock activity and thereby optimize agitation in accordance with papermaking conditions to provide a product of acceptable quality. It also offers the possibility that a contact element with a given profile can be vertically adjusted to be out of contact with the forming fabric and a support element with a different profile vertically adjusted to be in contact with the forming fabric. It further offers the opportunity to change the support element so that one element profile can be changed for another while the element is not in contact with the forming fabric. Since the effect, if any, of adjusting a particular support element can only be determined after the paper making conditions have stabilized and product made under the new conditions retrieved from the paper making machine and examined, there inevitably is a time lag between a support element being adjusted and the consequences of that adjustment being known. This time lag generally will be at least several minutes, but can be up to an hour.